A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g., comprising part of, one, or several dies) on a substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned.
Lithography is widely recognized as one of the key steps in the manufacture of ICs and other devices and/or structures. However, as the dimensions of features made using lithography become smaller, lithography is becoming a more critical factor for enabling miniature IC or other devices and/or structures to be manufactured.
A theoretical estimate of the limits of pattern printing can be given by the Rayleigh criterion for resolution as shown in equation (1):
                    CD        =                              k            1                    *                      λ            NA                                              (        1        )            
where λ is the wavelength of the radiation used, NA is the numerical aperture of the projection system used to print the pattern, k1 is a process dependent adjustment factor, also called the Rayleigh constant, and CD is the feature size (or critical dimension) of the printed feature. It follows from equation (1) that reduction of the minimum printable size of features can be obtained in three ways: by shortening the exposure wavelength λ, by increasing the numerical aperture NA or by decreasing the value of k1.
In order to shorten the exposure wavelength and, thus, reduce the minimum printable size, it has been proposed to use an extreme ultraviolet (EUV) radiation source. EUV radiation is electromagnetic radiation having a wavelength within the range of 5-20 nm, for example within the range of 13-14 nm. It has further been proposed that EUV radiation with a wavelength of less than 10 nm could be used, for example within the range of 5-10 nm such as 6.7 nm or 6.8 nm. Such radiation is termed extreme ultraviolet radiation or soft x-ray radiation. Possible sources include, for example, laser-produced plasma sources, discharge plasma sources, or sources based on synchrotron radiation provided by an electron storage ring.
EUV radiation may be produced using a plasma. A radiation system for producing EUV radiation may include a laser for exciting a fuel to provide the plasma, and a radiation source for containing the plasma. The plasma may be created, for example, by directing a laser beam at a fuel, such as particles of a suitable material (e.g., tin), or a stream of a suitable gas or vapor, such as Xe gas or Li vapor. The resulting plasma emits output radiation, e.g., EUV radiation, which is collected using a radiation collector. The radiation collector may be a mirrored normal incidence radiation collector, which receives the radiation and focuses the radiation into a beam. The radiation source may include an enclosing structure or chamber arranged to provide a vacuum environment to support the plasma. Such a radiation system is typically termed a laser produced plasma (LPP) source.
EUV collector mirrors, including grating collector mirrors are metal or metalloid or ceramic substrates coated with a molybdenum-silicon multi-layer stack for EUV radiation reflection purposes. The molybdenum-silicon multi-layer stack may be exposed to volatile species that are generated in the source, such as hydrogen ions. Continued exposure to volatile species such as hydrogen ions may result in the unintentional inclusion of hydrogen within the molybdenum-silicon layers. The unintentional inclusion of hydrogen may lead to the formation of blisters on the surface of the molybdenum-silicon multi-layer stack. The formation of such blisters can degrade the optical properties of the mirror, for example by reducing the reflectively, or altering the local focusing characteristics.
It is known to provide a capping layer at the surface of a molybdenum-silicon multi-layer stack, so as to block any ions from reaching the molybdenum-silicon layers. However, the use of a capping layer tends to reduce the reflectivity of the mirror. A further alternative is to increase the roughness at the interface between adjacent molybdenum-silicon layers. However, while having some beneficial effect by reducing the unintentional inclusion of hydrogen ions, this also reduces the reflectivity of the mirror surface.
What is needed is a multi-layer mirror that is resistant to blister formation during use in an EUV source.